The operators of communication networks such as, for example, the conventional telephone network, often provide the subscribers or customers, apart from the transmission of useful data (of voice in the telephone network), with a multiplicity of further facilities or subscriber performance features. These features include, e.g. conference circuits, the transmission or suppression of directory numbers, call forwarding or the charge pulse.
In the case of complaints by the subscriber or also in the case of a routine check of these assured features, the electrical characteristics of the subscriber lines (TAL), among other things, must be measured by the operator of the telephone network. During such a measurement, however, connected terminals must not audibly respond.
Two important electrical parameters of the subscriber line are the ohmic leakage resistance between line and ground and the so-called transmission-line constant, the capacitance between line and ground.
FIG. 1 then shows by way of example the basic diagrammatic structure of a current analogue subscriber connection of a telephone network.
In this arrangement, the respective subscribers TN are connected to a subscriber line module SLM allocated to the switching equipment via the corresponding subscriber line TAL. The subscriber line TAL itself usually consists of conventional twin copper wires, the two terminals being designated as a wire and b wire, respectively.
The circuit of the terminal connected to the subscriber line TAL corresponds to a capacitance C connected between the ends of the a wire and b wire in almost all terminals. This capacitance C is also called ringer capacitance since it is the component of the terminal which acquires the alternating voltage applied to the subscriber line TAL by the switching center in the event of an incoming call, that is to say which detects the incoming call.
Furthermore, FIG. 1 shows two leakage resistances Ra and Rb which represent the connections for the leakage currents between the two wires a and b of the subscriber line and ground.
In a convention method for determining these leakage resistances Ra and Rb and of the transmission-line constant (RC measurement), a problem occurs: in the current method, the ringer capacitance C also falsifies the measured values of the leakage currents on the wires (a and b) of the subscriber line TAL.
This measuring method according to the prior art will be described in the text which follows, with reference to FIG. 2.
FIG. 2 represents by way of example the sequence in time of a method for determining both the resistance between line and ground and the capacitance between line and ground on the a wire of a subscriber line of an analogue telephone connection.
During an RC measurement of the a wire, the second wire b is initially switched to high impedance. Following this, a voltage having the variation with time shown in FIG. 2 is applied to the line wire a to be measured.
In this process, a constant (in the present example negative) voltage UaG is first applied to the a wire. During this phase (I), the ohmic leakage current IRa1 between wire a and ground is measured.
Following this, a constant rising voltage UaG is applied to the a wire (phase II). During this second phase, the variation of the capacitor current ICa1 is measured. During the measurement, the voltage UaG applied to the b wire constantly rises from the negative area up to a predeterminable positive value.
After this maximum value has been reached, the voltage UaG is then kept constant (phase III). During the third phase, too, the ohmic leakage current is now measured. Thus, a second value is obtained for the leakage current, called IRa2 in this case.
To conclude the measurement, the applied voltage UaG is constantly reduced in phase IV. During this process, the current variation of the capacitive current of the subscriber line, here called Ica2, is measured a second time. Starting from a predetermined positive value, the voltage UaG then decreases to a predetermined negative value.
From these four values IRa1, IRa2, ICa1 and Ica2 detected during the measurement, leakage resistance and transmission-line constant of the a wire of the subscriber line can then be determined. To determine the corresponding values for the second wire b of the subscriber line, the steps described above are carried out with correspondingly exchanged starting positions.
As already mentioned above, the disadvantageous factor in the method described above for RC measurement is the fact that the ringer capacitance arranged between the two wires a and b falsifies the values for IRa1 and IRa2, respectively.